The invention relates to limiter circuits or zero-crossing detectors which transform an analog input waveform into a binary digital waveform which is suitable for interface to a signal processor, such as a microprocessor. More particularly, the invention relates to a limiter circuit with dynamic hysteresis which provides a clean limited output signal that has a wide input dynamic range and quick response to changes in DC level of the input waveform.
Text book circuits for limiters (see Burr-Brown "Operational Amplifiers" pages 358-364) solved the problem of glitches caused by noise or harmonic distortion at signal transition points by adding positive feedback or hysteresis to the comparator stage of the limiter. A "glitch" is defined by the IEEE Standard Dictionary of Electrical and Electronics Terms (1972) as a perturbation of the pulse waveform of relatively short duration and of uncertain origin. For specific applications a hysteresis level of a certain percentage of the input peak to peak swing voltage is usually found to be optimal. Currently, limiter circuits have hysteresis loops that are at a fixed level. As the input voltage level goes up or down the percentage of hysteresis with respect to the input voltage level changes so that the improvement provided by the hysteresis is effective only over a narrow range of input voltage levels. Accordingly, there is a need for a limiter circuit which solves the problem of glitches at the limiter output during signal transitions without being limited to operation over a narrow range of input voltage levels.
It is an object of the invention is to provide a limiter circuit with a hysteresis level that is a fixed percentage of the limiter input voltage level.
It is also an object of the invention to provide a limiter circuit with a hysteresis level that is effective over a wide range of input voltage levels.